Excimer lasers are of a class of ultra violet gas lasers with applications in scientific, medical and industrial markets.
The gas mixtures used in excimer lasers contain gases which create contaminants. The gas mixture of such a laser may have as its main constituents,
i) a halogen donor (typically 5 millibars of hydrogen chloride or fluorine). Henceforth in this specification and in the appended claims, the halogen donor is referred to simply as `halogen`, PA1 ii) an active rare gas (typically 200 millibars of argon, krypton or xenon), PA1 iii) an inert gas diluent (typically 3000 millibars of helium or neon).
Halogens are extremely corrosive, tending to corrode the materials of the laser interior to form contaminant compounds. Once contaminated the laser gas mixture has to be replaced. Because the mixtures are made up from relatively expensive rare gases, gas lifetime is important and, as the gas mixture becomes contaminated, laser performance is impaired. Contaminants can build up over a relatively short period, hours or days, depending on the amount of use the laser is subjected to, and replacing the gases is expensive and the laser usually has to be put out of commission whilst this is being done. This is particularly significant in industrial markets where downtime can add considerably to process costs.
Considerable efforts have been made to extend the lifetime of excimer laser gases and to identify the contaminants which are created during operation to facilitate their removal. (See, for example, UK Patent No 2,126,327).
One of the methods which has been developed for removing contaminants from an excimer laser gas mixture, involves passing the laser gas over a surface which reacts chemically with the contaminants thereby leaving them on the coated surface as a solid residue.
Although the inert components of the gas mixture do not react with this cleaner, the halogen donor does and, as a result, it also is completely removed from the gas mixture. Steps therefore have to be taken to replace the halogen after passage through the chemical cleaner.
Another often more effective way of removing these contaminants is to condense them on to a cold surface. A cryogenic condenser is disclosed in UK Patent No 2,126,327 (referred to above) which uses liquid nitrogen as its operating medium but provides a range of temperatures above the temperature of the liquid nitrogen, selectively to condense different contaminants. The laser gas circulates continuously through the condenser during laser operation.
During maintenance of the laser, by allowing the contaminants to warm up to ambient temperature so that they become gaseous, they can be disposed of as waste gases. The rare gases and the halogens have significantly high vapour pressures at those temperatures at which most of the contaminants can be condensed out of the gas mixture and they do not condense.
In normal operation the output of an excimer laser gradually deteriorates due to the build up of contaminants and the loss of halogen which combines with various materials to form the contaminants.
In order to compensate for this loss of halogen, a further technique which is used is to inject additional halogen into the laser system at regular intervals. However, this has the effect of producing a sudden increase in the laser output because the proportion of halogen in the system is suddenly increased, but it does not prevent the subsequent steady fall in output after the injection of the halogen and a further injection of halogen then become necessary. The steady fall in output can be compensated by gradually increasing the operating voltage of the excimer laser but this effects adversely the other laser parameters such as beam quality, pulse duration and beam uniformity. In any critical industrial process such changes are unacceptable.
The long term performance and hence usefulness, of an excimer laser is therefore degraded by,